A BRIEF ENCAPSULATION OF USER VISUAL NEEDS

This comes from the AG (WCAG) list. Passing along for consideration.

---------- Forwarded message ---------
From: Andrew Somers <notifications@github.com>
Date: Tue, Sep 17, 2019 at 9:08 PM
Subject: Re: [w3c/wcag] Proposal for color and contrast (1.3.1, 1.4.1,
1.4.3., 1.4.6, 1.4.11) (WCAG 3.0) (#901)
To: w3c/wcag <wcag@noreply.github.com>
Cc: Subscribed <subscribed@noreply.github.com>

<snip>
A BRIEF ENCAPSULATION OF USER VISUAL NEEDS: *Visual Acuity deficits:*

Acuity is essentially the ability to resolve a stimuli in the eye and
perceive it in focus. “Blurryness” is the plain language way to describe
poor acuity.

   - A _primary_way to assist visual acuity is corrective refraction
   (glasses/contacts) which is outside scope. In terms of display or design,
   and for all other things being equal, acuity is assisted by the
   appropriate _SIZE_which needs to be within a range (not too small but also
   not too big) for best perception.
   - Some causes of acuity loss, such as cataracts, require surgery to
   correct.
   - Classification of Acuity can be divided into three broad groups:
      - *20/10 thru 20/63:* normal through near-normal. Existing standards
      tend to be built around this range, which relates to a font size
of 12pt on
      the printed page. This serves as a “baseline” or foundation from which
      stronger accessibility needs can be defined. 20/30 is the lowest
acuity for
      a private pilot, and 20/40 is the lowest for non-commercial
drivers in most
      states.
      - *20/70 thru 20/200:* Low Vision, per the WHO definition. If a font
      at 100% size is good for 20/63, then if you double the size to
200% (24pt),
      you accomodate 20/150. To accomodate 20/200, then increase size to 275%
      (33pt).
      - *Above 20/200:* Legally blind. 20/400 needs 550% larger size (66pt).
   - *Discuss* size adjust (user) and design minimums. And accommodating
   user changes without breaking content, etc. (methods).

*Contrast Sensitivity deficits:*

Contrast Sensitivity Function (CSF) can be impacted by poor acuity, by
retinal disease such as AMD, retinal migrains, by degraded ocular media
(cataract, etc), and by neurological problems (MS, neuropathy). *VERY ROUGH
(to be written):*

   - CSF deficits caused due to poor acuity (blurry vision) is typically
   helped best by addressing the acuity issues when possible.
   - CSF is directly linked to spatial frequency (i.e. size), especially
   closer to threshold.
   - Increasing stimulus size will increase perceived contrast (within a
   range).
   - A key aspect of stimulus size is the stroke width of a font (i.e. font
   “weight”) — Increasing a font’s size increases perceived contrast, but
   largely due to the increase of stroke width as rendered to the screen.
   Stroke width is the aspect of a font that most closely follows Michelson
   Contrast (gratings).
   - Aging ocular media (lens, cornea, vitreous) can affect contrast, but
   moreover these can cause problems with glare which reduces perceived
   contrast, while simultaneously being made worse as stimulus contrast
   increases.
      - Intraocular glare reduces or obscures perceived contrast, but
      contrast perception is improved by _reducing the contrast _of
what is being
      viewed.
      - Put another way, higher contrast objects cause more glare which
      reduces the “contrast legibility” versus lower contrast objects
that cause
      less glare. The extreme example is headlights from an oncoming
car at night.
   - *TBD Discuss: luminance contrast, threshold vs supra and critical
   contrast levels. Discuss design contrast. Discuss display luminance adjust
   (user). Discuss polarity.*
   - Contrast Sensitivity Function is typically measured with a
   Pelli-Robson style of chart, which measures the “just noticeable
   difference” or threshold of visibility.
      - A Pelli-Robson score of 2 indicates “perfect normal vision
      contrast” which equates to a contrast of 1% (i.e. 1.01 to 1 )
      - A score of 1.5 is a noticeable degrading of CSF, and equates to a
      contrast of 3% (i.e. 1.03 to 1)
      - A score of 1 is a serious contrast impairment, and equates to a
      threshold contrast of 10% (1.1 to 1)
      - These are a measure of the point where a stimuli becomes visible,
      which is useful in a clinical setting for detecting disease, but do not
      indicate the level of “critical contrast” where an item is “most
readable.”

*Visual Field deficits:*

Closely related/essentially part of contrast sensitivity impairments are
those relating to visual field.

   - Central vision loss is a loss of vision in the fovea (central vision)
   forcing these users to learn to read using their peripheral vision.
   - Peripheral blindness, or narrowing of the visual field (aka tunnel
   vision),
      - Makes it harder to notice changes in content (i.e. a warning
      message) outside of the area the user is looking directly at.

*Color Vision deficits:*

Color Vision Deficiency (protan, deutan, tritan CVD types) is primarily
helped by ensuring there is enough luminance contrast between items (i.e.
between text and a background, or between roadways on a map and geographic
features on the map).

   - Also, ensure that color is not used as the sole means of providing
   information (that is, don’t rely on “red” as a color that means “stop” —
   descriptive text of symbols are also needed to communicate meaning.)
   - Protanopia (red deficient) may have problems with some monitor types
   (such as UHD/Rec2020) the red primary is close to the cut off for the green
   cone and is perceived much darker.. *(need plain language for this)*
   - sRGB monitors are recommended for Protanopia as the red primary is
   within the green cone sensitivity. The protan will see this red a little
   darker, which should be considered in calculating contrast.
   - The rare monochromats are also aided by luminance contrast, though may
   need to set the display to a monochrome mode, and have control over
   luminance and ambient illumination (such as for rod monochromacy).

*Cognitive/Neurological related Visual Deficits:*

62% of the brain is involved in visual processing. Over 20% of the brain is
dedicated to visual processing, and of 42% processes visual in conjunction
with other senses such as auditory and tactile.

   - The other impairment types above are mostly associated with the eye
   itself, these are associated with processing the signals from the eye.
   - Someone who had a stroke, and the stroke damaged some part of vision
   processing may have a problem with only that aspect of vision. For
   instance, if the motion detection part of the brain is damaged, they may
   see a car that is parked, but when the car moves it “disappears” in that
   the brain ignores it/it is nor “perceived” (“ *visual neglect* ”)
   - With *agnosia,* the visual pathways and brain are capable
   of _seeing_objects or people, but cannot _recognize_them.
   - Cognitive impairments, brain damage (from stroke or other incident)
   can also cause some of the functional problems normally associated with
   ocular impairments, such as  blurred vision, field loss, light sensitivity,
   hallucinations, etc.
   - Ocular migraines can directly interfere with vision by introducing
   “blockage” to vision, such as with ocular migraines auras, which can appear
   as zig zags in the vision, “seeing stars”, etc.

In Closing

Some of things you mention in your initial issue are fairly well
understood, and in fact making their way into either SC extensions for 2.2,
or new standards and guidelines for Silver. (One example is font weight, as
those proposed SCs are already being created).

However, you make statements that are not supported by research, your
codepen notwithstanding. Visual perception is not binary logic, so
"absolute" statements don't really fly in a field where there *are no
absolutes.* Human perception is far more complex than can be determined by
some examples at maximum contrast.

I have listed references and footnotes to authoritative research supporting
most of my posts in #695 <https://github.com/w3c/wcag/issues/695> and
elsewhere, and I do suggest reading through those references to gain a
better understanding of the underlying concepts. In particular you might
want to read Legge's book Psychophysics of Vision.

And please keep in mind these standards are ultimately about the functional
needs of a very wide swath of users & impairments. It is needs that should
be considered, not so much the abstraction layer methods.

Regards,

Andy

Andrew Somers
WAI Invited Expert


*Color Science Research Silver Task Force Low Vision Task Force*

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Jim Allan, Accessibility Coordinator
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"We shape our tools and thereafter our tools shape us." McLuhan, 1964